Method and solution for producing chromate coatings on zinc and zinc alloys



A. M. TRIGGLE ETAL 3,090,710 METHOD AND SOLUTION FOR PRODUCING CHROMATE COATINGS oN ZINC AND ZINC ALLoYs Filed May 25, 1960 May 21, 1963 N M H n uw G V www n F a m a N w R H -l -I f 5 m MN w n ow w a H O H .if :nm y il- J G w. m @c F United States Patent O Yori;

Fiied May 25, 1989, Ser. No. SL-tl Claims priority, application Great Britain May 27, 1959 7 llaims. (Ci. 14S-6.2.1)

i The present invention relates to an improved chromating solution, and more particularly, to a novel replenishable chromating solution Afor zinc and zinc alloy surfaces containing appropriate quantities of Ihexavalent and trivalent chromium at a ratio within a selected range.

It has long been known that surfaces of Zinc and zincbased alloys can be protected against corrosion by treatment with an acid solution containing hexavalent chromium. The protection is believed due to the formation of m amorphous film on the surface by chemical reaction between the chromium-containing solution and the zinc. It was found that attack of the solution on the surface was facilitated if the solution initially contained a small amount of trivalent chromium, and it has been proposed to introduce this by adding a compound of trivalent chromium, or preferably by adding a small amount of a suitable reducing agent. As these solutions are used more trivalent chromium is also formed by reduction of hexavalent chromium at the Zinc surfaces, so that the concentration of trivalent chromium progressively increases and the solution eventually has to be discarded owing to deterioration of the quality of the coating.

It has now been found that if the total chromium content of the solution is limited to a lower value than has hitherto been customary and a substantial proportion of the chromium is initially present in the trivalent state the concentration of both hexavalent and trivalent chromium decreases in use so that replenishment is possible by addition of appropriate amounts of hexavalent and trivalent chromium, While satisfactory protective coatings are obtained in the cold.

It is accordingly the primary object of the ypresent invention to lprovide a chromating solution for zinc and zinc alloy surfaces having a novel composition enabling the solution to be replenished during use by the addition of appropriate quantities of trivalent and hexavalent chromium.

In accordance with the practice of the present invention, an acid solution for the treatment of zinc and zinchased alloys is prepared by dissolving in water chromium trioxide (Cr03) and chromium sulphate having from 6 to 7 molecules of water of crystallisation (Cr2(SO4)3, 6-7 H2O) and has a total content of hexavalent chr-omium (CrW), calculated as CrO3, and trivalent chromium (Crm), calculated as anhydrous Cr2(SO4)3, of less than about 30 gm./l. but at least about 2 gin/l., 'while the ratio of hexavalent to trivalent chromium calculated in the same way, ranges from about 0.2 to about 7.0 by weight. All figures for chromium concentrations and ratios in this specification and in the subjoined claims refer to values calculated in this Way.

Solutions prepared in this way have a pH ranging from asstra Patented May 21, 1933 about 1.8 to about 2.8. Above or below this range little or no coating is obtained, and the solutions should therevfore be maintained within it.

It is an imprtant feature of the invention that the solutions used can be replenished. The concentration of both hexavalent and trivalent chromium in the solutions falls with use, the latter since more trivalent chromium is consumed in the coating reaction and lost by drag-out than is formed by reaction with the zinc or zinc alloy. The replenishment material should therefore include both CrO3 `as the source `of hexavalent 'chromium and Cr2(SO4 )3, 6-7 H2O as the source of trivalent chromium. The amounts and proportions `of these additions required for replenishment of any given solution are readily determined by analysis of the solution. |The pH of the solution generally tends to rise as the coating proceeds, and a small `addition of acid, preferably sulphuric acid, will therefore usually also be necessary to adjust the pH. Hydrochloric or nitric acids should not be added for this purpose, since the presence of appreciable amounts of chloride or nitrate ions in the solution leads to\ unsatisfactory coatings.

The nature of the compounds used both to make up and to replenish the solutions is very important. The

' use of a mixture of sodium or potassium chromate or bichromate and sulphuric acid in place of the CrO3 is not satisfactory, as the presence of appreciable amounts of sodium or potassium in the coating solution gives rise to brown, nonadherent coatings. The use of compounds of trivalent chromium other than Cr2(SO4) 3, 6-7 H2O is also unsatisfactory. Although chromium sulphate may be associated in the solid state with from 3 to 18 molecules of water per molecule, we iind that hydrates other than the hexahydrate, :heptahydrate or an intermediate hydrate are either insoluble or sparingly soluble, or if they are soluble part of the chromium is associated in solution with sulphate, or masked, and not available yfor coating. The statements about trivalent chromium in this specification refer only to unmasked trivalent chromium.

Of course the Cr2(SO4)3, 6-7 H2O need not be introduced into the solutions in the solid state, but may be dissolved in water separately and introduced as an aqueous solution. One such solution contains approximately 50% by weight -of Cr2(SO4)3, 6-7 H2O. Preferably the solutions are made up initially by diluting with water an aqueous concentrate containing both CrO3 and C-12(SO4)3 in which the ratio `of hexavalent to trivalent chromium ranges from about 0.2 to about 7'by weight.

Although appreciable amounts of sodium, potassium, chloride and nitrate are harmful, trace amounts can be tolerated without any great ill effects resulting. Nevertheless, care should be taken to maintain the solutions as free as possible from these ions, and preferably the only anions present are chromate and sulphate.

The solutions may be applied by any one of the conventional methods, such as, 'for example, spraying, brushing, immersion, etc., and are intended for use at room temperature, (about 60 R), coatings being obtained with processing times usually between lO and 60 seconds. However, temperatures up to about F. may also be used with correspondingly shorter processing times, but above this temperature a dusty coating or no lcoating at all is obtained.

The color of the coating varies with numerous factors, including the total chromium concentration in the solution, the ratio of hexavalent to trivalent chromium, the time and temperature of treatment and the nature of the surface being coated. A typical color pattern obtained on eleetrolytically galvanized surfaces using a standard processing time of 30 seconds at room temperature 1s shown in the accompanying drawing. The color intensity in general increases `with both the total chromium content and the proportion of hexavalent chromium. At total chromium concentrations less than 9 gm./l. with CrVrCrIII ratios less than 1.5, i.e. Within the area vCD35 on the drawing, the coatings obtained under these conditions are substantially colorless. A preferred solution for obtaining a substantially colorless coating is represented by the point A on the :drawing and contains 8.6 gm./l. of total chromium and has a CrVzCrIII ratio of 0.3.

vImproved corrosion resistance is obtained with colored coatings, particularly those having a deep Ayellow color, and for optimum corrosion resistance we prefer to use solutions represented by the area GHI] on the drawing and containing from to 20 gm./l. of total chromium and having a CrVTzCrIII ratio of from v1 to 3. A preferred solution for optimum corrosion resistance contains 10 gm./l. Cr03 and 7 grrL/l. Cr2(SO4)3 (anhydrous) and corresponds to the point B in the drawing.

Increase of the time of treatment above 30 seconds causes the color pattern to shift diagonally toward the origin of the graph and the saine effect is produced by increasing the temperature above room temperature. Decrease in the time of .treatment causes a shift diagonally outward away from the origin.

The solutions according to the invention can be used to give chromate coatings on all the main types of zinc material, including galvanized iron electrolytically galvanized iron, zinc foil and nine-base die castings.

To more fully illustrate the present invention, the following examples are provided by Way of illustration and are not intended yto be limiting in any way.

EXAMPLE I Lbs. Cr2(SO4)3 (calculated as anhydrous) 332 CrO3 150 Water to make 1,000

The pointage of the solution was determined in the usual way as the volume in milliliters of N/ 10 NaOH needed to neutralize a 10 ml. portion of the solution to a phenolphthalein end-point. The total chromium content of the solution was 5.2. gm./l. and the CrVzCrIII ratio was 0.3.

The solution was used at a temperature of 80-100 F., and the tubes were still warm from the galvanizing treatment, their temperature being about 130 F. The solution :was allowed to remain in contact with the tubes for 30 seconds, and the tubes were then rinsed with water and dried by a blast of air.

The pointage of the solution was determined from time to time, and was maintained at 6 points by the addition of a concentrated replenishing solution prepared from Cr03, Cr2(SO4)3, 6-7 H2O and sulphuric acid to have the composition:

EXAMPLE II 'Ihis is an example of the production of a colored coating.

A concentrated aqueous solution of CrO3 and Cr2(SO4)3, 6-7 H2O having the composition:

Lbs.

cro3 160 Cr2(SO4)3 (calculated as anhydrous) 112 f Water to make 1,000

was ldiluted with water to give a Working solution containing 10 gm./l. of chromic acid and Cr2(SO4)3, 6-7 H2O equivalent to 7 gm./l. of anhydrous chromium sulphate. Steel panels 4 X 2" in size, some coated with electrodeposited zinc and some galvanized by the Sendzimir process, were solvent-cleaned by trichlorethylene or white spirit and immersed in the solution at room temperature for 30 seconds. f

The panels were air-blasted to remove surplus water and oven dried at 250 F. The panels were subjected to corrosion tests by spraying at room temperature with a 20% salt solution and to a humidity test at 45 C. and humidity. The time in hours in each .test to the onset of white corrosion (white rust) (initial breakd0wn) and the occurrence of white corrosion over the whole surface area (complete lbrealcdown) is shown in the following Tables I and II:

Table I RESULTS OF SALT SPRAY TEST Time to Initial Time to Com- Surface Condition Breakdown plete Break- (hrs.) down (hrs.)

Hot-dip galvanized:

Bar Less than 20.-, 60.

Coated Electrodeposited:

Bare

What is claimed is:

l. A chromating Isolution consisting essentially of water, chromium trioXide, and Cr2(SO4)3, 6-7 H2O, said solution containing a total content of hexavalent chromium calculated as CIOB, and trivalent chromium calculated as yanhydrous Cr2(SO4)3, ranging `from about 2 to about 30 g'm./l., the weight ratio of said hexavalent chromium to said trivalent chromium ranging from about 0.2 to about 7.0, said solution having a pH ranging yfrom about 1.8 to about 2.8.

2. A chromating solution consisting essentially of water, chromium trioXide, and Cr2(SO4)3, 6-7 H2O, said solution containing a total content of hexavalent chromium calculated as CrO3, and trivalent chromium calculated as lanhydrous Cr2(SO4)3, ranging from about 2 to about 9 grrr/1., the weight ratio of said hexavalent chromium to said trivalent chromium ranging from about 0.2 to about 1.5, said solution having a pH ranging from about 1.8 to about 2.8.

3. A chromating solution consisting essentially of Water, chromium trioxide, and Cr2(SO4)3, 6-7 H2O, said solution containing a total content of hexavalent chromium calculated as CrO3, and trivalent chromium calculated as anhydrous Cr2(SO4)3, ranging from about 10 to about 20 gm./l., the Weight ratio of said hexavalent chromium to i said trivalent chromium ranging from about 1 to about 3,

said solution having a pH ranging from about 1.8 to about 2.8.

4. A chromating solution consisting essentially of water, chromium trioXide, and Cr2(SO4)3, 6-7 H2O, said solution containing a total content of hexavalent chromium calculated as Cr03, and trivalent chromium calculated as anhydrous Cr2(SO4)3 of about 8.6 gm./l., the weight ratio of said hexavalent chromium to said trivalent chromium of about 0.3, said solution having a pH ranging from about 1.8 to about 2.8.

5. A chromating solution consisting essentially of Water, chromium trioxide, and Cr2(SO4)3, 6-7 H2O, said solution containing a total content of hexavalent chromium calculated as CrO3, and trivalent chromium calculated as anhydrous Cr2(SO4)3 of about 17 gm./l., the Weight ratio of said hexavalent chromium to said -trivalent chromium of about 1.4, said solution having a pH ranging from about 1.8 to 4about 2.8.

6. The method of producing a chromate coating on zinc and zinc alloy surfaces comprising fthe steps of applying to said surfaces a solution at a temperature ranging from about room temperature to about 150 F. consisting essentially of water, chromium trioxide, and 1Cr2(SO4)3, 6-7 H2O, said solution containing a total content of heXav-alent chromium calculated as lCrOa, :and trivalent chromium calculated `as anhydrous C-r2 (S04) 3, ranging from about 2 to about 30 gm./l., the Weight ratio of said hexavalent chromium to said trivalent chromium ranging from about 0.12 to about 7.0, said solution having a pH ranging from `about 1.8 to about 2.8, and continuing the application of said solution until a coating results.

7. A concentrate suitable for dilution with Water for preparing and replenishing the solution having a composition according to claim 1 consisting essentially of an aqueous solution of chromium Vtrioxide and Cr2(SO4)3, 6-7 H2O, the weight ratio of said chromium trioXide to said Cr2(SO4)3, 6-7 H2O calculated as anhydrous Cr2(SO4)3 ranging from about 0.2 to about 7.0.

References Cited in the ile of this patent UNITED STATES PATENTS 2,021,592 Dubpernell et al Nov. 19, 1935 2,035,380 Wilhelm Max'. 24, 1936 2,138,794 Nelson et al. Nov. 29, 1938 2,338,924 Frasch Ian. 11, 1944 2,902,394 Jeremas Sept. l, 1959 FOREIGN PATENTS 589,887 Great Britain July 2, 1947 

1. A CHROMATING SOLUTION CONSISTING ESSENTIALLY OF WATER, CHROMIUM TRIOXIDE, AND CR2(SO4)3, 6-7 H2O, SAID SOLUTION CONTAINING A TOTAL CONTENT OF HEXAVALENT CHROMIUM CALCULATED AS CRO3, AND TRIVALENT CHROMIUM CALCULATED AS ANHYDROUS CR2(SO4)3, RANGING FROM ABOUT 2 TO ABOUT 30 GM./1., THE WEIGHT RATIO OF SAID HEXAVALENT CHROMIUM TO SAID TRIVALENT CHROMIUM RANGING FROM ABOUT 0.2 TO ABOUT 7.0, SAID SOLUTION HAVING A PH RANGING FROM ABOUT 1.8 TO ABOUT 2.8. 